1. Field of the Invention
The present disclosure relates to a nitride semiconductor device capable of serving as a power switching element.
2. Description of the Related Art
Gallium nitride (GaN) has a bandgap of 3.4 eV which is about three times wider than Si having a bandgap of 1.1 eV. Therefore, GaN has a high breakdown voltage. In addition, GaN has a high electron saturation velocity, so it has an excellent feature that a high-frequency operation can be performed. Especially, an ON-resistance (Ron) while a field effect transistor (FET) is in an ON-state can be considerably low compared with that of Si-based FET at the same breakdown voltage. As a result, a cooling system can be expected to be simpler, smaller, and lighter, so that a power element can be expected to be extremely low in power loss and used as an alternative to a Si power element. At the present, a metal-oxide semiconductor (MOS) FET or an insulated gate bipolar transistor (IGBT) made of Si is used as a power device, but its performance is approaching theoretical limitations, so that a power device made of GaN which is superior in material value to Si is drawing attention.
When the GaN power device is used as a key component in a power conversion control device such as an inverter, an energy-saving effect can be achieved in consumer equipment as one of energy-saving effects. When an operation frequency is increased and a switching operation is performed at high speed in the power conversion control device, a peripheral component such as an inductor can be small in size, and a power supply circuit can be miniaturized. However, in this case, a switching loss is increased, so that improvement in efficiency at a high frequency of 100 kHz or more is limited in the conventional Si power semiconductor. Meanwhile, when the GaN power device which has a high electron saturation velocity and a high breakdown electric field strength is used, a drift region can be reduced in size, and not only a switching loss but also a conduction loss can be reduced, so that even when a high-speed switching operation is performed, the loss can be lower than that of the Si power device.
For purpose of improving switching characteristics of the GaN device, there is a need to prevent a current collapse which is caused by a considerable reduction in drain current when a high drain bias is applied, and an increase in ON-resistance.
As a mechanism of the current collapse, it is considered that electrons accelerated by a high intensity of electric field, which become hot electrons, are trapped in an electron trap due to a crystal defect or a surface level, so that the drain current is reduced and the ON-resistance is increased. The increase in ON-resistance causes the switching characteristics to deteriorate, so that preventing the current collapse is indispensable in order to increase an output and reduce a loss in a GaN power device.
For purpose of preventing the current collapse in the GaN device, it is disclosed that a light emitting diode (LED) is formed for light irradiation, on a back surface or a front surface of an FET (refer to Unexamined Japanese Patent Publication No. 2006-286746). Furthermore, it is a reported that an FET and an LED for irradiating the FET with light beams are formed on a chip, and the FET is irradiated with the light beams to prevent the current collapse (refer to Unexamined Japanese Patent Publication No. 2008-47767).